Earless seals, also called “true seals" or "phocids", are the most diverse and widespread pinnipeds. They lack external ears, have more streamlined snouts, and are generally more aquatically adapted than otariids. They swim with efficient, undulating whole-body movements using their more-developed rear flippers. The swimming efficiency and an array of other physiological adaptations make them better built for deep and long diving and long distance migration. These mammals are, however, very clumsy on land, moving by wriggling their front flippers and abdominal muscles. The two back flippers form a tail-like structure which does not aid walking on land. True seals generally communicate by slapping the water and grunting, rather than vocalizing.

Eared seals, also called "walking seals" and "otariids", include the animals commonly known as sea lions and fur seals. These are vocal, social animals that are somewhat better adapted to terrestrial habitats with rear flippers that can turn forward so that they can move on all fours on land. Their foreflippers are larger than those of earless seals and are used as a primary source of maneuverability in the water. Eared seals have external ears, as their name suggests, and more dog-like snouts, further distinguishing them from true seals. While sea lions are generally larger than fur seals and lack the dense underfur of the latter, the long-standing division into subfamilies (Arctocephalinae and Otariinae for fur seals and sea lions respectively) is unjustified in light of genetic evidence suggesting that several fur seal species are more closely related to some sea lions than other fur seals. The iconic ball-balancing circus seal is generally some species of sea lion, most commonly a California Sea Lion.

The walrus is an exclusively Arctic species—the sole surviving member of the once diverse and widespread Odobenidae family. They are easily recognized by their long tusks and great bulk Template:Kg to lb. While they share with otariids the ability to turn their rear flippers forward, their swimming is more reminiscent of that of true seals, relying more on sinuous whole body movements. They also lack external ears. Unlike eared seals and true seals, which feed primarily by hunting fish and squid in the water column, walrus generally prefer benthicinvertebrates, in particular clams. The unique squirt and suck method of feeding on mollusks first differentiated the original walrus ancestor from other pinniped lineages. There remains debate as to whether the walrus diverged from the eared seals before or after the true seals.

Pinniped limbs, which have evolved into flippers, are proportionally shorter than those of most other mammals. Their fingers are bound together by a web of skin (as are its toes). They also have claws either on the front flippers (earless seals) or back flippers (eared seals). Because water has a much higher density than air, their flippers can be much smaller proportionately in relation to their size than bird or bat wings. Additionally, pinnipeds are essentially weightless in water, allowing them to come to a standstill, and perform aquabatic feats in water that would be impossible for flying creatures.

Fur seals have both blubber and a specially adapted fur coat, including outer guard hairs that repel water and a layer of insulating underfur. For this reason they were particularly prized by sealers. Many species were nearly hunted to extinction.

For most pinniped species molting is an annual process of replacing worn fur (and in some cases, skin) that temporarily grounds them. While molting, thermoregulation can be compromised, so some species, such as Elephant seals, fast and remain onshore for a month or more.

In many species, pups are born with a natal coat of a different length, texture and/or color than adults. This coat is adapted for the terrestrial, pre-weaning period, either a thick pelage to keep them warm in arctic environments, or a thin layer of fur to keep them cool on summer sands. During their first molt (about 11 days after birth for harp seals) the pups replace this with an adult coat better suited to life at sea. Until this age, pups risk hypothermia and drowning if they spend much time in the ocean.

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Pinnipeds use several strategies to conserve body heat while foraging in cold waters. Most pinnipeds primarily rely on a thick layer of blubber under their skin, which also provides buoyancy, hydrodynamic shape, and stored energy. Some young seals have a thick furcoat as well as blubber, dropped after 11 days for harp seals normally. Additionally, the pinniped circulatory system is uniquely adapted so that blood can be directed away from body surface areas to prevent heat loss.

Pinnipeds living in warmer climes, such as Galapagos or Australian sea lions, must keep cool when they haul out onto land to rest, breed, and nurse their pups. Strategies include resting in the shade or in tide pools, covering themselves in a thin layer of sand ("sand-flipping"). Their unique circulatory system also shunts blood to the surface of their flippers, which can then be rapidly cooled by waving or dipping in pools.

Pinniped eyes are well adapted for seeing both above and below the surface. When diving the animal has a clear membrane that covers and protects its eyes. In addition, its nostrils close automatically. Testicles and mammary glands are located in slits under the skin to maintain the pinniped’s streamlined shape. They also have whiskers to help navigate and sensors in their skull to absorb sounds underwater and transmit them to the cochlea.

Males of many species, (e.g. Elephant seals, South American Sea Lions, and Northern Fur Seals) aggressively defend groups of specific females, referred to as harems. Males of other species (e.g. most sea lions and Brown Fur Seals) defend territories on reproductive rookeries while females move freely between them. Possibly violent competition for females or territories is an integral part of male breeding strategy among most pinnipeds. Otariids, which are generally more land-adapted, form major aggregations in the summer months on beaches or rocky outcrops. Consequently, their reproductive behavior is easier to observe and consequently well-studied. Walruses and many phocids, on the other hand, form smaller aggregations, often in remote locations or on ice, and copulate in the water. Their reproductive behavior is less well known.

Females have a postpartumoestrus that allows them to mate soon after giving birth. Delayed embryo implantation (embryonic diapause) obviates the need to come ashore (haul-out) twice, once to give birth and again to mate. After giving birth, mothers suckle their young for a variable period. Phocid lactation varies from 4 to 50 days, whereas otarids lactate from 4 to 36 months. This reflects the fact that phocid feeding grounds tend to be far off-shore, so lactation is associated with maternal fasting. To compensate for the short lactation period, the fat content of phocid milk (45–60% fat) is higher than in any other marine mammal species. After lactation, most females migrate to feeding grounds for intensive foraging to recoup energy reserves. Otariid feeding grounds are generally closer to shore and mothers take foraging trips. Otariid milk fat content is lower than that of phocids, owing to the protracted lactation period (typically 25–50%). Protracted nursing also leads to the formation of social bonds.

Pinnipeds can hold their breath for nearly two hours underwater by conserving oxygen.[1] When the animal starts diving its heart rate slows to about one-tenth of its normal rate. The arteries squeeze shut and the sense organs and nervous system are the only organs that receive normal blood flow. Pinnipeds are able to resist more pain and fatigue caused by lactic acid accumulation than other mammals. However, once they return to the surface, they need time to recover and normalize their body chemistry.[2]

Sexual coercion is extremely common among pinnipeds, even on other species. In one incident that was caught on videotape, a 100 kg Antarctic fur seal pinned down a 15 kg king penguin and thrusted its pelvis, trying to insert its penis into the penguin for about 45 minutes.[1]

Recent molecular evidence suggests that pinnipeds evolved from a bearlike ancestor about 23 million years ago during the lateOligocene or early Miocene epochs, a transitional period between the warmer Paleogene and cooler Neogeneperiod.[4]
The earliest fossil pinniped that has been found is Puijila darwini, of about 23 million years ago. Pujilla had heavy limbs, indicative of upright movement on land, and flattened phalanges, indicating that they were probably webbed, but not yet flippers. The discovery of Pujilla in northern Canada strongly suggests that pinnipeds originated in the Arctic. The reference to Charles Darwin is in honor of his contention made in On the Origin of Species (1859) that

A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted in an animal so thoroughly aquatic as to brace the open ocean[5]

Another (more advanced) early pinniped is Enaliarctos, which lived 24 – 22 million years ago. It is believed to have been a good swimmer, but to have been able to move on land as well as in water, more like an otter than like modern pinnipeds. There has been longstanding debate as to whether walruses diverged from a common otariid-phocid ancestor, or whether the phocids diverged before a common otariid-odobenid ancestor. The most recent evidence suggest that the latter hypothesis is more likely.[7]